1. Field of the Invention
The present invention relates generally to a method for using an IP (Internet Protocol) address, and in particular, to a method for using a unique IP address in a private IP address domain.
2. Description of the Related Art
In general, a data packet is comprised of transmission data, at least one header and a footer of a lower layer, the header and the footer surrounding the transmission data. First, a header (of an application layer) for identifying an application is attached to the data, and then, a header (of a transport layer) for identifying a port and a protocol is attached to the resulting header-attached data. A network layer identifies the devices in the transmission side and the reception side using a network address such as an IP address. Finally, data link layer information is included in the data packet. In an Internet application, the network layer includes IP addresses of the transmission side device and the reception side device, and for correct transmission, these addresses should be unique addresses. However, because of the limitation on the number of possible addresses, it is not possible to allocate the unique IP address to every device on the Internet. One method for reducing a demand for the IP addresses is to use network address translation (NAT). In order to use NAT, it is necessary to allocate unique IP addresses to the devices connected to the Internet through a gateway such as a router. For example, the device connected to the Internet through a local area network (LAN) and the router, has an IP address that is unique only between the device and the router, and is not unique on the Internet. In the following description, the device having a unique IP address will be referred to as a “LAN device” for convenience of explanation.
A brief description of the NAT will be made below. In Internet communication, the respective LAN devices have different IP addresses, thus making it possible to identify the devices. However, not having a unique IP address, the LAN device cannot transmit a packet directly to the reception side over the Internet communication. Even though the packet is directly transmitted to the reception side, the device cannot receive a response to the transmitted packet. Instead, when it is desired to transmit a packet, the device includes its non-unique IP address, i.e., a source address (SA) in the network layer header and then transmits this packet to the router. The router removes this SA and instead, inserts its unique IP address, i.e., a router source address (RSA). The router registers SA, DA (Destination Address), SP (Source Port), DP (Destination Port) and PT (Protocol Type) of the network layer and the transport layer in a packet, and then, transmits the packet to the reception side over the Internet. Every response from the reception side includes SA2, DA2, SP2, DP2 and PT2. When the packet has been transmitted by the router, SA2 has a value of DA and DA2 has a value of RSA. The router searches the LAN address of the device that has transmitted this packet, by consulting the first registered SA, DA, SP, DP and PT, changes the reception address of the packet from RSA to SA, and then, transmits the changed address to the LAN device.
Although it is possible to solve the IP address limitation problem to some extent using the above method, there are some applications it cannot be used in. This is because upon failure to receive a response to an IP allocation request, it is not possible to transmit a packet to the device connected to the LAN through the Internet. Further, since the transmission side can transmit the packet only with the unique IP address, it is only possible to transmit the packet using the router.
However, the router does not know to which LAN device this packet should be transmitted.
In addition, a problem occurs even when an application transmits its IP address to the other party. When the LAN device transmits a LAN IP address, the reception side cannot transmit a response to the transmission side using this address. As one example of this case, there is an ITU (International Telecommunication Union) H.323 protocol. In the ITU H.323 protocol, the transmission side transmits an IP packet with its own IP address to the other party, and the reception side performs communication with the transmission side using this address. In this case, the LAN device cannot receive a response from the reception side, because the LAN device knows only the LAN IP address. In order to make such communication available, the router must change the IP address of the LAN device to the unique IP address by examining every packet, and must also perform a reverse operation thereof. This overloads the router.
Meanwhile, when the LAN device desires to statically use the unique IP address, the router can statically allocate one unique IP address to a specific LAN device. In this case, the router performs only an operation of converting the LAN IP address of the packet to the unique IP address and converting the unique IP address to the LAN IP address.
As stated above, when the device connected to the LAN is not allocated the unique IP address, it cannot receive a response from the other party even though the application transmits its own IP address to the other party. Further, the router must convert the IP address by examining every packet.